Carburetor

ABSTRACT

In an air valve carburetor, a vent switch valve responsive to the pressure in the mixture conduit downstream of the air valve vents fuel vapor from the fuel bowl alternatively to the mixture conduit air inlet and to a fuel vapor storage canister. Fuel vapor is purged from the canister to a port in the mixture conduit adjacent and traversed by the upstream edge of the air valve.

States Patent 11 1 Dec.3,1974

[21] Appl. No.1 343,554

[52] [1.8. CI. 261/50 A, 261/72 R, 261/D1G. 67 [51] Int. Cl. F02m 5/08 [58] Field of Search 26l/DIG. 67, 72 R, 50 A;

3,540,423 ||/l )7() 'l'ullcs 123/136 3,548,797 12/1971) Hugihnru c1111. 1, 26l/Dl(i. 67 3,618,578 11/1971 Swatmun 1. 123/136 3,683,597 8/1972 Bevcridgc et a1. 55/316 FOREIGN PATENTS OR APPLICATIONS 1,280,617 10/1968 Germany 261/D1G. 67

Primary ExaminerTim R. Miles Attorney, Agent, or Firm-C. K. Veenstra 5 7 ABSTRACT In an air valve carburetor, a vent switch valve responsive to the pressure in the mixture conduit downstream of the air valve vents fuel vapor from the fuel bowl alternatively to the mixture conduit air inlet and [56] ReferencesCited 2 to a fuel vapor storage canlster. Fuel vapor 1s purged UNITED STATES PATENTS from the canister to a port in the mixture conduit ad- 3,198,498 8/1965 Mennesson 261/144 jacent and traversed by the upstream edge of the air 3,322,408 5/1967 St0ltman..... va1ve 3,392,965 7/1968 Harrison 261/50 A 3,460,5 22 8/1969 Kittler et .61 .1 261/D1G. 67 8 Claims, 4 Drawmg Flgures "-1 56 58 r\ as: i/

This invention relates to air valvecarburetors and, more particularly, to systems in such carburetors for venting fuel vapor from the fuel bowl alternatively to the mixture conduit air inlet and to a fuel vapor storage canister and for purging fuel vapor from the canister.

In recent years, many vehicles have been equipped with canisters which receive and store fuel vapor emitted from the fuel tank. During engine operation, the fuel vapor has been purged into the air cleaner or carburetor at various points along the induction air flow path.

This invention provides an air valve carburetor with provision for purging fuel vapor into the mixture conduit through a port adjacent and traversed by the upstream edge of the air valve. The relatively constant subatmospheric pressure below the air valve induces a flow of purge air and fuel vapor through the purge passage from the fuel vapor storage canister. This purge flow is increased from a minimum when the port is upstream of the air valve to a maximum when the port has been traversed by the air valve.

In some of the aforementioned vehicles, the fuel vapor storage canister also received fuel vapor from the carburetor fuel bowl. The fuel vapors were vented from the fuel bowl alternatively to the canister or to the carburetor mixture conduit air inlet past a vent switch valve operated both bymanifold vacuum and by linkage from the throttle.

This invention also provides an air valve carburetor with a vent switch valve responsive to the pressure in the mixture conduit downstream of the air valve. That pressure, which is substantially constant during engine operation, controls the vent switch valve without requiring supplemental control by throttle-connected linkage.

The details as well as other objects and advantages of this invention are set forth in the remainder of the specification and are shown in the drawings in which:

FIG. I is a sectional elevational view of the carburetor showing the basic metering linkage;

FIGS. 2 and 3 are enlarged views of the metering rod showing the configuration of the tapered portion; and

FIG. 4 is a view similar to FIG. I showing the vent switch valve and fuel vapor storage canister purge systems.

Referring first to FIG. I, the carburetor has a mixture conduit 12 including an air inlet I4 and a mixture outlet I6 which discharges to the engine. A throttle I8 is disposed in mixture outlet I6 in the usual manner on a throttle shaft 20.

An air valve 22 is disposed in air inlet 14 on an air valve shaft 24. A spring 26 is hooked over the downstream edge 28 of air valve 22 and extends to a bracket 30 to bias air valve 22 to the position shown.

A tang 32 reaches upwardly from air valve 22 and is connected by a link 34 to a diaphragm 36. A chamber 38, formed between the right side of diaphragm 36 and a cover member 40, is connected by a tube 42 to a region 44 of mixture conduit 12 defined between air valve 22 and throttle l8.

A chamber 46, defined between the left side of diaphragm 36 and a cover member 48, is subjected to substantially atmospheric pressure, present in air inlet 14 and in the air cleaner (not shown), through openings such as 50, 52 and 54. (The air cleaner seats on a rim 56 disposed about the upper portion of carburetor 10.)

In operation, chamber 38 is subjected to the subatmospheric pressure created in region 44 as throttle I8 is opened, and diaphragm 36 acts through link 34 to pull air valve 22 clockwise to an open position. Spring 26 is effective to balance the opening force of diaphragm 36, thereby creating a substantially constant subatmospheric pressure in region 44. By thus establishing a generally constant pressure drop across air valve 22, the area about air valve 22 and thus the rotative position of air valve 22 is determined by and is a measure of the rate of air flow through mixture conduit 12.

A tab 58 extends upwardly from air valve 22 and is connected through a link 60 to one end 62 of a lever 64. The opposite end 66 of lever 64 is pivoted about a pin 68. Intermediate ends 62 and 66, a hanger 70 extends from lever 64 into the carburetor fuel bowl 72. The lower end 74 of hanger 70 has a hook 76 which is receivedin a recess 78 formed in a metering rod 80.

It may be noted that hanger 70 extends through an opening 82 in the cover 84 for fuel bowl 72. Opening 82 is closed by a slider 86 which shifts horizontally during movement of hanger 70.

Metering rod is disposed in a fuel passage 88 having its lower end 90 disposed to receive fuel from a well 92 formed in the bottom of fuel how] 72. The upper end 94 of fuel passage 88 has an opening 96 through which fuel is discharged into region 44 of mixture conduit 12. It will be appreciated, therefore, that the fuel in fuel bowl 72 is subjected to a substantially constant metering head from the substantially atmospheric pressure in the upper portion of the fuel bowl to the generally constant pressure in region 44.

A metering jet or orifice 98 is disposed in fuel passage 88 around the tip 99 of metering rod 80. As best shown in FIGS. 2 and 3, metering rod-80 has flat tapered surfaces 100 on opposite sides which, upon reciprocation of metering rod 80 in jet 98, varies the area available for fuel flow through jet 98.

In operation, as air valve 22 opens by clockwise rotation, link 60 rotates lever 64 in a clockwise direction. Lever 64 then lifts hanger 70 to move metering rod 80 generally upwardly and rightwardly in fuel passage 88. Thus as air valve 22 is opened to increase the area available for air flow through air inlet I4, metering rod 80 is shifted to increase the area available for fuel flow through metering orifice 98. By this means, a substan tially constant air-fuel ratio may be maintained the precise proportion being controlledby the geometry of tapered surfaces 100 and of the linkage between air valve 22 and metering rod 80. V

A' spring 102 extends from an annular ledge 104 formed in fuel passage 88 to the lower end 106 of metering rod 80 to take up any slack in the linkage and to load metering rod 80 against jet 98.

It may be noted from FIG. 3 that the thickness of metering rod 80 increases from the end of surfaces I00 most closely adjacent passage inlet. 90 to tip 99. Tip 99 is therefore enlargedand assists in discharging fuel from fuel passage 88 as air valve 22 and metering rod 80 are moved to increase air. and fuel flow. This offsets the greater inertia of the fuel which otherwise could create a mixture temporarily leaner than desired.

Referring now to FIG. 4, fuel bowl 72 is vented through a passage 110 formed in cover member 84. Passage 110 extends to a vent chamber 112 formed between a lower member 114 and an intermediate member 116. A branch passage 118, formed in lower member 114, extends past a valve seat 120 to a valve chamber 122 formed between lower member 114 and fuel bowl cover member'84. A vertical passage 124 opens from valve chamber 122 to a horizontal passage 126 containing a tube fitting 128. A tube 130 extends from fitting 128 to a fuel vapor storage canister 132, such as that disclosed in U.S. Pat. No. 3,683,597, for example. By this means, fuel vapor evolved in fuel bowl 72 when the engine is not operating is directed to canister 132 for storage.

A signal passage 134 extends from a port 136 in region 44 to a chamber 138 defined between a diaphragm 140 and an upper member 142. During engine operation, the substantially constant subatmospheric pressure created in region 44 is'adequate to overcome the bias of a spring 144 or other device such as a weight and lift diaphragm 140 awayfrom a valve seat 146. At the same time, a valve stem 148 secured to diaphragm 140 lifts a valve disc 150 away from a valve seat 152 i into engagement with valve seat 120.

Thus during engine operation, fuel vapor evolved in fuel bowl 72 may not be directed from vent chamber 112 through branch passage 118 as that route is blocked by engagement of valve disc 150 with valve seat 120. instead fuel vapor is directed from vent chamber 112 through a branch passage 154 past valve seat 146 into a chamber 156 formed between diaphragm 140 and intermediate member 116. Chamber 156 is open through one or more apertures 159 to the region within air cleaner rim 56, and thus as indicated by the broken line, fuel vapor is directed to the mixture conduit air inlet 14 for delivery to the engine. By this means, also, the pressure in fuel bowl 72 above the fuel level is equalized with that in mixture conduit air inlet 14 as is usually required for accurate fuel metering.

It is noted that valve disc 150 is slidably mounted on valve stem 148 and is biased toward valve seat 152 by a spring 160. This construction permits simultaneous engagement of valve disc 148 against valve seat 152 and diaphragm 140 against valve seat 146 without requiring close tolerances therebetween.

As noted above, valve disc 150 is disengaged from valve seat 152 during engine operation. This opens a passage 162 (coaxialwith branch passages 154 and 118) from valve chamber 122 to a horizontal passage 164. Passage 164 is coaxial with passage 126 and is separated therefrom by a plug 166 seated against a shoulder 168. Passage 164 extends to a passage 170 which opens through a port 172 to mixture conduit 12. Thus during engine operation, the subatmospheric pressure in mixture conduit 12 draws air through fuel vapor storage canister 132, and the air and purged fuel vapor are drawn through tube 130, fitting 128, passages 126 and 124, valve chamber 122, valve seat 152, passages 162, 164 and 170, and port 172 to mixture conduit 12 for deliveryto the engine. When air valve 22 is in the closed or idling position shown, port 172 is disposed completely upstream of air valve 22 and the rate of flow of purge air and purged fuel vapor is at a minimum; in'many applications the depression upstream of air valve 22 (caused by the air cleaner) may be insufficient to cause any purge flow. As air valve 22 is opened, the upstream edge 174 traverses port 172 to gradually decrease the pressure at port 172 and thereby gradually increase the purge flow rate. After edge 174 of air valve 22 has completely traversed port 172, port 172 is subjected to the substantially constant subatmospheric pressure in region 44, nd a substantially constant purge flow rate is thereby provided.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An internal combustion engine carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream from said air valve during engine operation, a fuel bowl containing fuel up to a certain level therein, a fuel vapor passage opening from said fuel bowl above said level, said fuel vapor passage having a first branch extending to said mixture conduit and a second branch, valve means disposed in said vapor passage for alternatively directing the flow of fuel vapor through said first and second branches, said valve means including a diaphragm responsive to the pressure in said region to position said valve means for directing the flow of fuel vapor from said fuel bowl through said first branch to said mixture. conduit during engine operation and for directing the flow of fuel vapor from said fuel bowl through said second branchwhen the engine is not operating.

2. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and opening into said region whereby the subatmospheric pressure therein may purge fuel vapor from said device into said region for delivery to the engine when the engine is operating, a fuel bowl containing fuel up to a certain level, a vent passage opening from said fuel bowl above said level, said vent passage having a first branch extending to said mixture conduit for delivering fuel vapor from said fuel bowl to the engine when the engine is operating and a second branch extending to said purge passage for delivering fuel vapor from said fuel bowl to said fuel vapor storage device when the engine is not operating, valve means disposed in said passage, means biasing said valve means to prevent the flow of fuel vapor through said first branch and to permit the flow of fuel vapor through said second branch, said valve means including a diaphragm responsive to the subatmospheric pressure maintained in said region during engine operation to overcome said biasing means to permit the flow of fuel vapor through said first branch and to prevent the flow of fuel vapor through said second branch during engine operation. 1

3. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said carburetor to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and extending to said mixture conduit, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower, intermediate and upper members secured to said cover member, vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and intermediate members, a first branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branch passage, a diaphragm clamped between said intermediate and upper members, a second branch passage extending from said vent chamber to a chamber defined between said diaphragm and said intermediate member, means equalizing the pressure in said last mentioned chamber with the pressure in said mixture conduit upstream of said air valve, a valve seat formed about said second branch passage and engaged by said diaphragm to prevent flow therethrough, valve seats formed in said valve chamber about said first branch passage and said purge passage, a valve stem secured to said diaphragm and extending through said branch passages into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagement with said valve seats formed therein, means biasing said diaphragm into engagement with said second branch passage valve seat and said valve disc with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said diaphragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said diaphragm is disengaged from said second branch passage valve seat and said valve disc is disengaged from said purge passage valve seat and engaged with said first branch passage valve seat.

4. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit and rotatable between closed and open positions, means controlling said carburetor to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and having a port disposed in said mixture conduit adjacent the upstream edge of said air valve when said air valve is in said closed position and traversed by the upstream edge of said air valve as said air valve is rotated from said closed position, whereby the pressure in said mixture conduit may induce a flow of fuel vapor from said storage device through said purge passage to said mixture conduit which may be increased from a minimum when said air valve is in said closed position to a selected value when the upstream edge of said air valve has traversed said port, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower, intermediate and upper members secured to said cover member, a vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and intermediate members, a first branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branch passage, a diaphragm clamped between said intermediate and upper members, a second branch passage extending from said vent chamber to a chamber defined between said diaphragm and said intermediate member, means equalizing the pressure in said last mentioned chamber with the pressure in said mixture conduit upstream of said air valve, a valve seat formed about said second branch passage and engaged by said diaphragm to prevent flow therethrough, valve seats formed in said valve chamber about said first branch passage and said purge passage, a valve stem secured to said diaphragm and extending through said branch passages into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagemnt with said valve seats formed therein, means biasing said diaphragm into engagement with said second branch passage valve seat and said valve disc with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said dia' phragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said diaphragm is disengaged from said second branch passage valve seat and said valve disc is disengaged from said purge passage valve seat and engaged with said first branch passage valve seat.

5. An internal combustion engine carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream from said air valve during engine operation, a fuel bowl containing fuel up to a certain level therein, a fuel vapor passage opening from said fuel bowl above said level, said fuel vapor passage having a first branch extending to said mixture conduit and a second branch extending to a fuel vapor storage device, valve means disposed in said vapor passage for controlling the flow of fuel vapor through said second branch, said valve means including a diaphragm responsive to the pressure in said region to position said valve means for restricting the flow of fuel vapor from said fuel bowl through said second branch during engine operation and for permitting the flow of fuel vapor from said fuel bowl through said second branch when the engine is not operating. g

6. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and opening into said region whereby the subatmospheric pressure therein may purge fuel vapor from said device into said region for delivery to the engine when the engine is operating, a fuel bowl containing fuel up to a certain level, a vent passage opening from said fuel bowl above said level, said vent passage having a first branch extending to said mixture conduit for delivering fuel vapor from said fuel bowl to the engine when the engine is operating and a second branch extending to said purge passage for delivering fuel vapor from said fuel bowl to said fuel vapor storage device when the engine is not operating, valve means disposed in said second branch, means biasing said valve means to permit the flow of fuel vapor through said second branch, said valve means including a diaphragm responsive to the subatmospheric pressure maintained in said region during engine operation to overcome said biasing means to restrict the flow of fuel vapor through said second branch during engine operation.

7. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said carburetor to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and extending to said mixture conduit, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower and upper members secured to said cover member, a vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and upper members, a branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branch passage, adiaphragm clamped between said lower and upper members, valve seats formed in said valve chamber about said branch passage and said purge passage, a valve stem secured to said diaphragm and extending through said branch passage into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagement with said valve seats formed therein, means biasing said valve disc into engagement with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said diaphragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said valve disc is disengaged from said purge passage valve seat and engaged with said branch passage valve seat.

8. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit and rotatable between closed and open positions, means controlling said carburetor to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and having a port disposed in said mixture conduit adjacent the upstream edge of said air valve when said air valve is in said closed position and traversed by the upstream edge of said air valve as said air valve is rotated from said closed position, whereby the pressure in said mixture conduit may induce a flow of fuel vapor from said storage device through said purge passage to said mixture conduit which may be increased from a minimum when said air valve is in said closed position to a selected value when the upstream edge of said air valve has traversed said port, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower and upper members secured to said cover member, a vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and upper members, a branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branchv passage, a diaphragm clamped between said lower and upper members, valve seatsformed in said valve chamber about said branch passage and said purge passage, a valve stem secured to said diaphragm and extending through i said branch passage into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagement with said valve seats formed therein, means biasing said valve disc into engagement with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said diaphragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said valve disc is disengaged from saidpurge passage valve seat and engaged with said branch passage 

1. An internal combustion engine carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream from said air valve during engine operation, a fuel bowl containing fuel up to a cerTain level therein, a fuel vapor passage opening from said fuel bowl above said level, said fuel vapor passage having a first branch extending to said mixture conduit and a second branch, valve means disposed in said vapor passage for alternatively directing the flow of fuel vapor through said first and second branches, said valve means including a diaphragm responsive to the pressure in said region to position said valve means for directing the flow of fuel vapor from said fuel bowl through said first branch to said mixture conduit during engine operation and for directing the flow of fuel vapor from said fuel bowl through said second branch when the engine is not operating.
 2. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and opening into said region whereby the subatmospheric pressure therein may purge fuel vapor from said device into said region for delivery to the engine when the engine is operating, a fuel bowl containing fuel up to a certain level, a vent passage opening from said fuel bowl above said level, said vent passage having a first branch extending to said mixture conduit for delivering fuel vapor from said fuel bowl to the engine when the engine is operating and a second branch extending to said purge passage for delivering fuel vapor from said fuel bowl to said fuel vapor storage device when the engine is not operating, valve means disposed in said passage, means biasing said valve means to prevent the flow of fuel vapor through said first branch and to permit the flow of fuel vapor through said second branch, said valve means including a diaphragm responsive to the subatmospheric pressure maintained in said region during engine operation to overcome said biasing means to permit the flow of fuel vapor through said first branch and to prevent the flow of fuel vapor through said second branch during engine operation.
 3. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said carburetor to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and extending to said mixture conduit, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower, intermediate and upper members secured to said cover member, vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and intermediate members, a first branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branch passage, a diaphragm clamped between said intermediate and upper members, a second branch passage extending from said vent chamber to a chamber defined between said diaphragm and said intermediate member, means equalizing the pressure in said last mentioned chamber with the pressure in said mixture conduit upstream of said air valve, a valve seat formed about said second branch passage and engaged by said diaphragm to prevent flow therethrough, valve seats formed in said valve chamber about said first branch passage and said purge passage, a valve stem secured to said diaphragm and extending through said branch passages into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagement with said valve seats formed therein, means biasing said diaphragm into engagement with said second branch passage valve seat and said valve disc with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said diaphragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said diaphragm is disengaged from said second branch passage valve seat and said valve disc is disengaged from said purge passage valve seat and engaged with said first branch passage valve seat.
 4. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit and rotatable between closed and open positions, means controlling said carburetor to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and having a port disposed in said mixture conduit adjacent the upstream edge of said air valve when said air valve is in said closed position and traversed by the upstream edge of said air valve as said air valve is rotated from said closed position, whereby the pressure in said mixture conduit may induce a flow of fuel vapor from said storage device through said purge passage to said mixture conduit which may be increased from a minimum when said air valve is in said closed position to a selected value when the upstream edge of said air valve has traversed said port, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower, intermediate and upper members secured to said cover member, a vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and intermediate members, a first branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branch passage, a diaphragm clamped between said intermediate and upper members, a second branch passage extending from said vent chamber to a chamber defined between said diaphragm and said intermediate member, means equalizing the pressure in said last mentioned chamber with the pressure in said mixture conduit upstream of said air valve, a valve seat formed about said second branch passage and engaged by said diaphragm to prevent flow therethrough, valve seats formed in said valve chamber about said first branch passage and said purge passage, a valve stem secured to said diaphragm and extending through said branch passages into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagemnt with said valve seats formed therein, means biasing said diaphragm into engagement with said second branch passage valve seat and said valve disc with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said diaphragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said diaphragm is disengaged from said second branch passage valve seat and said valve disc is disengaged from said purge passage valve seat and engaged with said first branch passage valve seat.
 5. An internal combustion engine carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream from said air valve during engine operation, a fuel bowl containing fuel up to a certain level therein, a fuel vapor passage opening from said fuel bowl above said level, said fuel vapor passage having a first branch extending to said mixture conduit and a second branch Extending to a fuel vapor storage device, valve means disposed in said vapor passage for controlling the flow of fuel vapor through said second branch, said valve means including a diaphragm responsive to the pressure in said region to position said valve means for restricting the flow of fuel vapor from said fuel bowl through said second branch during engine operation and for permitting the flow of fuel vapor from said fuel bowl through said second branch when the engine is not operating.
 6. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said conduit, means controlling said air valve to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and opening into said region whereby the subatmospheric pressure therein may purge fuel vapor from said device into said region for delivery to the engine when the engine is operating, a fuel bowl containing fuel up to a certain level, a vent passage opening from said fuel bowl above said level, said vent passage having a first branch extending to said mixture conduit for delivering fuel vapor from said fuel bowl to the engine when the engine is operating and a second branch extending to said purge passage for delivering fuel vapor from said fuel bowl to said fuel vapor storage device when the engine is not operating, valve means disposed in said second branch, means biasing said valve means to permit the flow of fuel vapor through said second branch, said valve means including a diaphragm responsive to the subatmospheric pressure maintained in said region during engine operation to overcome said biasing means to restrict the flow of fuel vapor through said second branch during engine operation.
 7. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit, means controlling said carburetor to maintain a region of substantially constant subatmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and extending to said mixture conduit, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower and upper members secured to said cover member, a vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and upper members, a branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branch passage, a diaphragm clamped between said lower and upper members, valve seats formed in said valve chamber about said branch passage and said purge passage, a valve stem secured to said diaphragm and extending through said branch passage into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagement with said valve seats formed therein, means biasing said valve disc into engagement with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said diaphragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said valve disc is disengaged from said purge passage valve seat and engaged with said branch passage valve seat.
 8. For use with an internal combustion engine having a fuel vapor storage device, a carburetor comprising a mixture conduit, an air valve disposed in said mixture conduit and rotatable between closed and open positions, means controlling said carburetor to maintain a region of substantially constant subaTmospheric pressure in said mixture conduit downstream of said air valve during engine operation, a purge passage adapted for connection to said fuel vapor storage device and having a port disposed in said mixture conduit adjacent the upstream edge of said air valve when said air valve is in said closed position and traversed by the upstream edge of said air valve as said air valve is rotated from said closed position, whereby the pressure in said mixture conduit may induce a flow of fuel vapor from said storage device through said purge passage to said mixture conduit which may be increased from a minimum when said air valve is in said closed position to a selected value when the upstream edge of said air valve has traversed said port, a fuel bowl containing fuel up to a certain level, a cover member secured over said fuel bowl, lower and upper members secured to said cover member, a vent passage extending from said fuel bowl through said cover member and said lower member to a vent chamber defined between said lower and upper members, a branch passage extending from said vent chamber to a valve chamber defined between said lower and cover members, said purge passage extending through said valve chamber and having a portion disposed coaxially with said branch passage, a diaphragm clamped between said lower and upper members, valve seats formed in said valve chamber about said branch passage and said purge passage, a valve stem secured to said diaphragm and extending through said branch passage into said valve chamber, a valve disc mounted on said valve stem and disposed in said valve chamber for alternative engagement with said valve seats formed therein, means biasing said valve disc into engagement with said purge passage valve seat, and a signal passage extending from said mixture conduit region to a chamber defined between said diaphragm and said upper member whereby during engine operation the bias of said biasing means is overcome and said valve disc is disengaged from said purge passage valve seat and engaged with said branch passage valve seat. 